This invention relates to a process and catalyst for reducing the aromatics and olefins content of hydrocarbon distillate products. More particularly, this process relates to an improved catalytic hydrogenation process and catalyst wherein the catalyst comprises platinum and palladium incorporated onto a support comprising beta zeolite and sodium.
For the purpose of the present invention, the term "hydrogenation" is intended to be synonymous with the terms "hydrotreating" and "hydroprocessing," and involves the conversion of hydrocarbons at operating conditions selected to effect a chemical consumption of hydrogen. Included within the processes intended to be encompassed by the term hydrogenation are aromatic hydrogenation, dearomatization, ring-opening, hydrorefining (for nitrogen removal and olefin saturation), and desulfurization (often included in hydrorefining). These processes are all hydrogen-consuming and generally exothermic in nature. For the purpose of the present invention, distillate hydrogenation does not include distillate hydrocracking which is defined as a process wherein at least 15% by weight of the distillate feedstock boiling at a temperature above 430.degree. F. at atmospheric pressure is converted to products boiling below 430.degree. F.
Petroleum refiners are now facing the scenario of providing distillate fuels, boiling in the range of from about 150.degree. F. to about 700.degree. F. at atmospheric pressure, with substantially reduced sulfur and aromatics contents. Sulfur removal is relatively well defined, and at constant pressure and adequate hydrogen supply, is generally a function of catalyst and temperature.
Aromatics removal presents a substantially more difficult challenge. Aromatics removal is generally a function of pressure, temperature, catalyst, and the interaction of these variables on the chemistry and thermodynamic equilibria of the dearomatization reaction. The dearomatization process is further complicated by the wide variances in the aromatics content of the various distillate component streams comprising the hydrogenation process feedstock, the dynamic nature of the flowrates of the various distillate component streams, and the particular mix of mono-aromatics and polycyclic aromatics comprising the distillate component streams.
The criteria for measuring aromatics compliance can pose additional obstacles to aromatics removal processes. The test for measuring aromatics compliance can be, in some regions, the FIA aromatics test (ASTM D1319), which classifies mono-aromatics and polycyclic aromatics equally as "aromatics." Hydrogenation to mono-aromatics is substantially less difficult than saturation of the final ring due to the resonance stabilization of the mono-aromatic ring. Due to these compliance requirements, hydrogenation to mono-aromatics is inadequate. Dearomatization objectives may not be met until a sufficient amount of the polycyclic aromatics and mono-aromatics are fully converted to saturated hydrocarbons.
While dearomatization may require a considerable capital investment on the part of most refiners, dearomatization can provide ancillary benefits. Distillate aromatics content is inextricably related to cetane number, the accepted measure of diesel fuel quality. The cetane number is highly dependent on the paraffinicity of molecular structures, whether they are straight-chain or alkyl attachments to rings. A distillate stream which comprises mostly aromatic rings with few or no alkyl-side chains generally is of lower cetane quality while a highly paraffinic stream is generally of higher cetane quality.
Dearomatization of refinery distillate streams can increase the volume yield of distillate products. Aromatic distillate components are generally lower in gravity than their similarly boiling paraffinic counterparts. Saturation of aromatic rings can convert these lower API gravity aromatic components to higher API gravity saturated components and expand the volume yield of distillate product.
Dearomatization of refinery distillate streams can also provide increased desulfurization and denitrogenation beyond ordinary levels attendant to distillate desulfurization processes. Processes for the dearomatization of refinery distillate streams can comprise the construction of a new dearomatization facility, the addition of a second-stage dearomatization step to an existing distillate hydrogenation facility, or other processing options upstream of distillate hydrogenation or at the hydrogenation facility proper. These dearomatization steps can further reduce the nitrogen and sulfur concentrations of the distillate component and product streams, thus reducing desulfurization and denitrogenation catalyst and temperature requirements in existing distillate hydrogenation facilities designed primarily for hydrorefining. Reduced distillate sulfur and nitrogen concentrations can additionally increase the value of these streams for use as blending stocks to sulfur-constrained liquid fuel systems and as fluid catalytic cracking unit (FCC) feed.
While distillate dearomatization can provide cetane number improvement, volume expansion, and additional desulfurization and denitrogenation, the process has seldom been attractive in view of the large capital costs, the fact that many refiners have not reached distillate cetane limitations, and the relatively low cost of cetane improving additives. Now that legislation exists and further legislation is being considered to mandate substantial reductions in distillate aromatics content, the demand for distillate dearomatization processes is now being largely determined by the incentive to continue marketing distillates.
The use of beta zeolite in catalyst supports for distillate dearomatization has met with limited success and is commercially rare. Beta zeolite is generally paraffin-selective, attacking the paraffinic components of a feedstock in preference to the aromatics so that when a feed containing both aromatic and paraffinic components is processed over zeolite beta, the paraffinic components are converted first with the aromatic components tending to remain until higher conversion is attained. This paraffin-selective behavior is described in U.S. Pat. No. 4,419,220 to Lapierre et al. For this reason, beta zeolite has traditionally been preferred for processes such as catalytic hydrodewaxing.
The use of beta zeolite with noble metals such as palladium and platinum for hydrogenation has been particularly rare since hydrogenation processes have historically emphasized desulfurization and denitrogenation. Transition metals such as cobalt, molybdenum, nickel, and tungsten have generally been preferred alternatives to the noble metals for desulfurization and denitrogenation.
For example, U.S. Pat. No. 5,011,593 to Ware et al. discloses a process for catalytically hydrodesulfurizing catalytically cracked feeds such as light cycle oils over a catalyst containing zeolite beta and transition metals such as cobalt and molybdenum. The hydrodesulfurization process is specifically designed for the desulfurization of feedstocks containing large concentrations of aromatics. Since aromatic saturation is not an objective of the process, Ware et al. notes that the desulfurization process can be effectively operated at low to moderate hydrogen pressures.
U.S. Pat. No. 5,011,593 to Oleck et al. discloses a process for hydrodewaxing heavy petroleum residual feedstocks over a catalyst containing zeolite beta and one or more metals from Group VIA and Group VIII of the Periodic Table such as iron, cobalt, and nickel. The Oleck et al. process utilizes a catalyst comprising 5 to 30 wt % of zeolite beta and having 75% of its pore volume in pores no greater than 100 Angstroms in diameter and 20% of its pore volume in pores greater than about 300 Angstroms in diameter.
Since the above processes are not particularly directed to the dearomatization of distillates, hydrogenation metals such as cobalt, molybdenum, nickel, and tungsten are preferred to noble metals such as palladium and platinum. As noted by Oleck et al., nickel and cobalt hydrogenation metals are generally preferred to the noble metals, and in particular, palladium and platinum, since palladium and platinum can be considerably less effective for desulfurization and denitrogenation.
The use of metal mixtures on a catalyst support has also been the subject of research. (See P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York 1967.) Rylander teaches that two platinum metal catalysts, when used together, can give better rates or better yields than either catalyst individually. However, except for certain selected examples, there seems to be no way of predicting when mixtures of catalysts will prove advantageous. A useful guide as to the probable effectiveness of coprecipitated metal catalysts, is the performance of a mechanical mixture of the two metals. (See Rylander, at pages 9-11.)
U.S. Pat. No. 3.943,053 to Kovach et al. discloses a hydrogenation process using a catalyst comprising a particular mixture of platinum and palladium on an inert oxide support such as beta, eta, or gamma alumina. The process provides gasoline and distillate hydrogenation, but with limited hydrogenation activity. The process avoids use of silica-alumina supports since use of silica-alumina in gasoline service can result in the conversion of high octane benzene into substantially lower octane cyclohexane.
More recently, platinum and palladium combinations have been utilized on molecular sieves for distillate hydrogenation.
U.S. Pat. No. 5,1 51,172 to Kukes et al. discloses a process for the hydrogenation of distillate feedstocks over a catalyst comprising a combination of palladium and platinum on a support comprising mordenite.
U.S. Pat. No. 5,147,526 to Kukes et al. discloses a process for the hydrogenation of distillate feedstocks over a catalyst comprising a combination of palladium and platinum on a support comprising zeolite Y with from about 1.5 wt % to about 8.0 wt % of sodium.
The above processes provide a substantial improvement in distillate dearomatization over the prior art desulfurization catalysts described above.
However, it has now been found that processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising beta zeolite, result in a further improvement in overall distillate hydrogenation compared to the prior art hydrogenation processes including processes having a catalyst incorporating platinum and palladium onto inert oxide supports such as alumina and onto molecular sieve-containing supports such as mordenite, zeolite Y, and borosilicate. This particular synergy is more profound (in contradistinction to the teachings of Rylander) since physical mixtures of platinum and palladium on a beta zeolite support have been shown not to provide improved hydrogenation.
It has also been found that processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising beta zeolite combined with a particularly targeted concentration of sodium result in substantially improved hydrogenation compared to prior art hydrogenation processes and to processes having a catalyst incorporating platinum and/or palladium onto a support comprising beta zeolite with lower or higher than the particularly targeted sodium levels.
It is therefore an object of the present invention to provide a process and catalyst that provide improved distillate aromatics saturation.
It is also an object of the present invention to provide a process and catalyst that provide improved distillate desulfurization and denitrogenation.
It is another object of the present invention to provide a process and catalyst that increase distillate cetane number.
It is yet another object of the present invention to provide a process and catalyst that expand the volume of the distillate feedstock.
Other objects appear herein.